Method of enhancing strength of paper products and the resulting products

Abstract

The invention is a method of enhancing the strength of paper products, particularly the dry strength, without adversely affecting repulpability. It is also directed to the resulting products. It is particularly applicable but not limited to products with significant amounts of secondary fiber in the furnish. Preferably, about 10-30% of the fiber is separated from the furnish at some point prior to sheeting. This is treated with a cationic wet strength resin which is allowed to bond to the fiber. Cationic polyamide-epichlorohydrin resins are particularly useful. The treated fiber is them mixed with the untreated balance of the fiber at some point before the paper machine. Screnning fines on repulping do not normally exceed 2-3%.

We claim: 
     
       1. A method for making a readily repulped cellulosic fiber paper product which comprises: separating from 5-40% of the fiber from the bulk of the fiber furnish;   treating the separated fiber in an aqueous suspension with 0.5-5.0% by weight of a cationic crosslinking-type wet strength resin additive for a sufficient time to permit bonding of the resin to the cellulosic fiber;   recombining and thoroughly and uniformly mixing the treated fiber with the untreated balance of the cellulosic fiber furnish in an aqueous slurry; and   sheeting and drying the mixed treated and untreated fiber into a paper product at a temperature sufficient to achieve at least partial crosslinking of the resin additive whereby dry strength of the product is enhanced without adverse effect on repulpability.   
     
     
       2. The method of claim 1 in which 10%-30% of the fiber is treated with the cationic resin additive. 
     
     
       3. The method of claim 1 in which there is a delay time of at least 30 seconds after addition of the resin to the separated fiber to permit bonding of the resin to the fiber before it is recombined with the untreated fiber. 
     
     
       4. The method of claim 3 in which the delay time is between 30 seconds and one hour. 
     
     
       5. The method of claim 1 in which the crosslinking-type wet strength resin is selected from the group consisting of urea-formaldehyde condensation products, melamine-urea-formaldehyde condensation products, and polyamide-epichlorohydrin reaction products. 
     
     
       6. The method of claim 5 in which the chemically reactive resin is a polyamide-epichlorohydrin reaction product. 
     
     
       7. The method of claim 6 in which the polyamide-epichlorohydrin resin is used in an amount sufficient to attain 0.1-0.6% by weight based on the recombined fiber in the paper product. 
     
     
       8. The method of claim 1 in which the cellulosic fiber furnish is predominantly never before dried virgin fiber. 
     
     
       9. The method of claim 1 in which the cellulosic fiber furnish contains at least 5% recycled previously dried fiber. 
     
     
       10. The method of claim 7 in which the cellulosic fiber furnish contains 10-100% recycled previously dried fiber. 
     
     
       11. A cellulosic fiber paper product which comprises from 5-40% of the fiber treated with 0.5-5.0% of a reactive crosslinking-type wet strength resin additive uniformly blended with 95-60% of untreated fiber, said resin being at least partially crosslinked, the paper product having increased dry strength and having improved repulpability compared with a similar product in which all of the fiber has been treated with an equivalent amount of the resin. 
     
     
       12. The paper product of claim 11 in which from 10-30% of the fiber has been treated with the resin additive. 
     
     
       13. The paper product of claim 11 in which the resin is selected from the group consisting of urea-formaldehyde condensation products, melamine-urea-formaldehyde condensation products, and polyamide-epichlorohydrin reaction products. 
     
     
       14. The paper product of claim 13 in which the resin is a polyamide-epichlorohydrin reaction product. 
     
     
       15. The paper product of claim 11 which contains from 0.1-0.6% by weight of the resin based on the total amount of cellulosic fiber in the product. 
     
     
       16. The paper product of claim 11 which comprises predominantly never before dried virgin cellulosic fiber. 
     
     
       17. The paper product of claim 11 which comprises a mixture of never before dried virgin fiber at least 5% recycled previously dried fiber. 
     
     
       18. The paper product of claim 17 in which comprises a mixture containing from 90-0% never before dried virgin fiber and from 10-100% previously dried recycled fiber.